We know the original intention re. function of young buildings like the Millenium Dome , Eiffel Tower , Angel of the North but that function has changed even since they were built .The Roundhouse stared off life as engine shed ,became a psychedelic temple and is now a music venue ,all in the space of a couple of generations . Megalithic monuments were in use ,in some cases for over a millennium did the later users know the original function if any ? So often what were to become imposing sites started off as a place for Mesolithic feasting then later there may have been some pits dug with some deposits of pottery maybe just pure charcoal , later still maybe some cremated bone was buried then surrounded by a kerb then a low cairn or even a much larger one complete with chambers or alternatively surrounded by standing stones . Often they simply evolved .Was the eventual form of Silbury in mind of the those that built the first low mounds ?
It’s difficult to say what many were because of the developments and hence there will always be a mystery , but sometimes we can say what they weren’t .